This invention relates to compounds useful for complexing halogens and more particularly to compounds useful as an electrolyte additive for electrochemical cells and batteries of the type which utilize a halogen as its electrochemically active agent.
Voltaic cells which include an aqueous solution of metal halide, e.g., cadmium or zinc halide, as an electrolyte are known, but are frequently characterized by a relatively high self-discharge rate, low capacity, and high internal resistance. Because elemental halogen is soluble in the aqueous electrolyte, it is difficult to keep metllic zinc or cadmium and elemental halogen apart while simultaneously achieving a system in which a good percentage of the theoretical energy storage capacity can be realized.
Various attempts have been made to prevent elemental halogen from migrating to the zinc or cadmium electrode. For example, U.S. Pat. Nos. 2,566,114 and 3,373,058 to M. R. Bloch teach the use of quaternary ammonium halides in zinc or cadmium halogen cells. However, the salts suggested for use in such cells exist as a crystalline material when complexed with halogen. While such crystalline quaternary ammonium halides are capable of reducing the concentration of dissolved halogen during cell charge and discharge, because of their crystalline nature, they are unable to rapidly complex and release halogen or to remain concentrated in the vicinity of the current collector during cell cycling.
U.S. Pat. No. 3,816,177 to M. A. Walsh teaches the use of soluble quaternary ammonium halides and the like which may be dissolved in the electrolyte together with a water soluble depolarizer. When elemental halogen is released into the electrolyte, it combines with the quaternary halide to form a quaternary polyhalide which in turn complexes with the depolarizer to form an insoluble, halogen rich, liquid complex. If an inert electrode made of a material which absorbs the insoluble complex is employed, an improved cell is provided because the liquid complex, being fluid, permits rapid diffusion of halogen during cell charge and discharge and because the complexed halogen molecules, being concentrated about the current collector, are available for electrochemical reaction to an improved degree. While this system represents a significant improvement over the use of crystalline quaternary polyhalides, it suffers from the requirement that the depolarizer, which cannot directly complex halogens, must be part of the liquid complex.
Further improvements in halogen electrochemical cells are disclosed in U.S. Pat. No. 4,038,459 to A. M. Ajami et al. entitled "Halogen Complexing Alcohols and Nitriles" and in U.S. Pat. No. 4,038,460 to F. M. Walsh et al. entitled "Halogen Complexing Ethers", both filed on Mar. 17, 1976. The watersoluble alcohols, ethers or nitriles disclosed in these applications form liquid polyhalides in the presence of elemental halogens or quaternary ammonium-halogen complexes which polyhalides are insoluble and halogen-rich. While cells and batteries containing these types of additives have certain advantages, these additives contribute significantly to the cost of cells containing them.
Another approach to the problem of improving the performance of halide cells is disclosed in U.S. Application Ser. No. 723,142, filed Sept. 14, 1976, by A. M. Ajami et al now Pat. No. 4,065,601. This application teaches the use of two phase electrolytes comprising an aqueous phase and a water immiscible organic phase. A halogen complexing organic salt such as an ammonium, pyridinum, sulfonium, or phosphonium salt is dissolved in the organic phase. This system depends on gravity separation of the phases of the electrolyte or on the ability of the halide electrode to absorb the organic phase.
A present concensus is that the use of liquid polyhalides in halogen cells constitute an improvement over the use of crystalline polyhalides. Liquid polyhalides are preferred because the halogen stored in the liquid polyhalide is more rapidly available for electrochemical reaction than would be the case if the polyhalide were stored in a crystalline matrix. Furthermore, liquid polyhalides can be transported to a separate storage location in the cell as they are formed electrochemically. From the storage location, the polyhalides can be rapidly returned and discharged at the bromine electrode or the polyhalide can be used to supply halogen to the electrolyte for discharge at the halogen electrode. This increased mobility of liquid polyhalides as compared to the crystalline polyhalides results in cells with increased storage capacity (through external storage), with improved electrode performance (through more rapid intraphase halogen transfer), and with increased ease in designing practical cells. Those skilled in the art recognize that cells which utilize liquid polyhalides can use electrode structures which are either porous or non-porous, which allow either halogen rich electrolyte or liquid polyhalide flow-by or flow-through, or which have the liquid polyhalide uniformly distributed on the halogen electrode surface by either physical or adsorption means.
Cells with crystalline polyhalides relay on their crystalline nature to maintain contact between the solid polyhalides and the halogen electrode. When such cells are in operation, nucleation sites on the halogen electrode provide a site for solid polyhalide crystal growth and adherence. However, even if the crystals are in close contact with the electrode, they cannot transfer halogen to or from the halogen electrode as rapidly as can a liquid polyhalide due to the decreased diffusivity of the halogen species in a crystalline polyhalide matrix compared to that in a liquid polyhalide matrix. Furthermore, certain cells utilizing crystalline polyhalides require a large powdered halogen electrode surface area against which to hold crystals. Such powdered electrode surfaces are not suitable for use with liquid polyhalides. Liquid polyhalides flow through the powdered electrode matrix during charge and poor halogen electrode performance is obtained during discharge because little of the powdered matrix is in intimate contact with the liquid polyhalide.
For proper operation, cells utilizing crystalline polyhalides must thus be maintained at temperatures less than the melting points of the crystalline polyhalides employed. Generally speaking, cells which utilize crystalline polyhalides, such as those disclosed in the Block patents, must be operated at a temperature less than 30.degree. C. in order to maintain the crystalline nature of the polyhalides. In contradistinction to the foregoing, cells employing additives in accordance with the present invention must be maintained at temperatures greater than 30.degree. C. in order to prevent the additives from crystallizing. During normal operation of the type of cell toward which the present invention is directed, the internal resistance of the cells during the charging cycle and the discharging cycle is normally sufficient to maintain the temperature of the additives at 30.degree. or greater. However, during periods when the cell is neither being charged nor discharged, it is necessary to heat the polyhalides to a temperature of 30.degree. or more to prevent the polyhalides from crystallizing. Thus, in addition to utilizing the polyhalide additives of the present invention, the present invention also includes a means for maintaining the temperature of the additives at 30.degree. C. or higher to prevent their crystallization.
The instant invention constitutes a further improvement in the halogen cell art and provides seven compounds, one or more of which may be added to the electrolyte of halogen cells of the type described. The compounds of the invention eliminate the requirement of adding a depolarizer or other additional additive to the electrolyte yet provide increased halogen complexing ability if maintained at 30.degree. C. or higher; and thus, enhance the shelf life and capacity of the cells and batteries in which they are used.